1. Field of the Invention
My invention relates generally to the field of treating ship ballast water containing potentially undesirable organisms, and more particularly to a method and apparatus for treating ship ballast water to kill potentially undesirable aerobic and/or anaerobic microorganisms in the water to prevent them from being transported from one coastal area to another.
2. Basic Conditions and Vocabulary
Oceangoing ships may transport various organisms contained in their ballast water from any port in the world to any other port in the world. Macroorganisms can be filtered out, and the remaining microorganisms can be of extremely diverse natures due to their diverse origins. The term colony forming unit (CFU) is often used to describe any microorganism that reproduces. Thus, unless otherwise modified, the terms, CFU and microorganism, shall have the same meaning. Levels of dissolved oxygen in a ship's ballast water play a part in my invention, and the abbreviation, DO, will be used to connote such dissolved oxygen.
3. Description of the Prior Art
When a ship unloads cargo in a foreign port, the resulting empty holds of the ship are often filled with the local water as ballast to stabilize the ship. When arriving at a U.S. port or other loading port to take on replacement cargo, the ship typically discharges the prior local, that has now become foreign, ballast water into coastal waters in or near the U.S. (or other) port, thereby introducing nonindigenous organisms, including aerobic and anaerobic microorganisms, which may have a deleterious (or at least an unknown) effect on the ecosystem of the receiving coastal waters.
One current proposal for attempting to cure this problem is to require ships to exchange, in mid-ocean or on the high seas, the original local ballast water with the saline, open ocean sea water; however, such an exchange of ballast water on the high seas is potentially extremely hazardous, especially, for large cargo ships and tankers, and in any event has been shown not to be fully effective in removing all microorganisms from a ship's holds.
This mid-ocean ballast exchange is usually (but not always) safe when the space in which the water is being exchanged is small enough so as not to create a dangerous instability or structural stress condition during the water-pumping process for effecting the exchange; for example, such a ballast exchange usually (but not always) is safe when dealing with specialized ballast tanks or other similar small spaces because of the low ratio of the weight of the involved water with the overall weight of the ship, and because of the ability of the load bearing strength of the ship to accommodate a temporarily empty "small" specialized ballast tank.
However, it is often required that cargo holds or large ballast tanks be filled with water so that the ship's center of gravity is lowered during a voyage when no cargo is carried. Under these conditions, one or more cargo holds or tanks are often filled with ballast water. Because these holds or tanks contain very large spaces, they must be kept either completely full or completely empty so that interior wave action does not develop. Thus, if ballast exchange were attempted from such a large space and the ship encountered heavy seas in the middle of the pumping process, then there could develop inside the space a wave action which could destabilize the ship and create a very dangerous condition. Capsizes and loss of life have been attributed to vessels encountering unexpected sea conditions during ballast exchange. Unless done with perfect skill in keeping the hull of a vessel in proper trim and balance, ballast exchange will also exert internal structural stress on the vessel, even in good weather, so as to damage the structure in such a way that a traumatic structural failure can occur either immediately or over time. Such stress is of the nature of bending the steel wire of a paper clip back and forth until it breaks.
That the problem of purging ship ballast water of nonindigenous microorganisms is a long standing one which has not been satisfactorily solved is well documented. For example, see: The Introduction of Nonindigenous Species to the Chesapeake Bay via Ballast Water, Chesapeake Bay Commission, Jan. 5, 1995; BIMCO Weekly News, No. 8, Feb. 19, 1997, The Baltic and International Maritime Counsel; "Push for Rules on Ballast Exchange Gains Support", the Journal of Commerce, Mar. 26, 1996; "Stemming the Tide of Change", The Journal of Commerce, Jun. 24, 1996; "Ballast Rule Ineffective for Pest Control in Lakes", The Journal of Commerce, Jun. 24, 1996; and "Governors Offer Grant to Fight Great Lakes Invaders", The Journal of Commerce, Jul. 26, 1996. Furthermore, the U.S. Congress recently passed the "National Invasive Species Act of 1996" (P.L. 104-332) which requires the U.S. Coast Guard to issue voluntary national ballast water guidelines within a year.
U.S. Pat. No. 5,192,451 discloses a method of controlling the growth of zebra mussels in ship ballast water by adding a polymer to the ballast water; however, the use of chemicals to treat ballast water, which has been discharged into U.S. coastal waters, may have an adverse environmental effect on the ecosystem. U.S. Pat. Nos. 5,376,282 and 5,578,116 disclose the use of a vacuum and agitation for reducing the dissolved oxygen of natural source water specifically to a level below that sufficient to support the survival respiration of zebra mussels; however, there is no disclosure of treating ship ballast water to oxygenate and then deoxygenate the water in a process which encompasses the general worldwide problem of the transfer from any one to any other coastal zone of microorganisms in the ballast water of a ship. U.S. Pat. No. 3,676,983 discloses an apparatus including a vacuum chamber and an agitator for removing gases from a liquid; however, there is no recognition of the problem of nonindigenous microorganism treatment in ship ballast water, and the removal of dissolved oxygen in the water to a level where aerobic microorganisms are killed. U.S. Pat. No. 4,316,725 discloses several methods, including the use of a vacuum, to remove dissolved oxygen from water. U.S. Pat. No. 3,251,357 discloses injecting combustion/stack gases into water for treating the water to inhibit the growth of, e.g., microorganisms; however, there is no recognition of, or suggested solution for, the long felt problem of how to remove anaerobic or aerobic microorganisms from ship ballast water.